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From: Mike Terry <news.dead.person.stones@darjeeling.plus.com>
Newsgroups: comp.theory
Subject: =?UTF-8?Q?Re:_Flibble=e2=80=99s_Leap:_Why_Behavioral_Divergence_Imp?=
 =?UTF-8?Q?lies_a_Type_Distinction_in_the_Halting_Problem?=
Date: Tue, 13 May 2025 22:45:27 +0100
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On 13/05/2025 22:22, Mike Terry wrote:
> On 12/05/2025 05:37, Keith Thompson wrote:
>> Richard Heathfield <rjh@cpax.org.uk> writes:
>>> On 12/05/2025 04:11, Keith Thompson wrote:
>>>> Richard Heathfield <rjh@cpax.org.uk> writes:
>>>> [...]
>>>>> ALL C compilers are required to diagnose ALL syntax errors and ALL
>>>>> constraint violations.
>>>> Yes, all conforming C compilers are required to do that.� (Well,
>>>> strictly speaking they're only required to issue at least one diagnostic
>>>> for any translation unit that violates a syntax rule or constraint.)
>>>
>>> I was unintentionally ambiguous, for which I apologise.
>>>
>>> The point I sought to make is that there is no syntax error (or
>>> constraint violation) so trivial that a compiler is given licence not
>>> to issue a diagnostic it if it has no other reason so to do.
>>>
>>> That is, they are all capable of ticking the box that says 'must issue
>>> at least one diagnostic'.
>>>
>>>> [...]
>>>>
>>>>> In my experience, Microsoft's C compiler - although not perfect - is
>>>>> pretty good at following conformance rules. I'd be surprised to learn
>>>>> from a competent source that it misses a syntax error.
>>>> I wouldn't, since few if any C compilers are conforming by default.
>>>
>>> I was talking about conforming mode, which IIRC (it's been a while) is
>>> invoked by -W4 (a warning level that I habitually used in the days
>>> when I still used Microsoft software).
>>>
>>>> I've just tried 4 different C compilers (gcc, clang, and tcc
>>>> on Ubuntu, MS Visual Studio 2022 on Windows), and none of them
>>>> diagnosed a stray semicolon at file scope *by default*.� gcc and
>>>> clang can be persuaded to diagnose it.� tcc, as far as I can tell,
>>>> cannot; I don't believe it claims to be fully conforming in any mode.
>>>> I wasn't able to get MSVS to diagnose it, but there could easily
>>>> be an option that I'm missing.
>>>
>>> Could you crank MSVS up to -W4 (or whatever the max is these days) and
>>> try again? I hate to impose, but of course it's your own fault for
>>> qualifying as a competent source. ;-)
>>
>> It's "/W4".� The default appears to be "/W3".
> 
> At W4 or Wall I get a messages such as:
> 1>c:\products\source\scratch10\ctest.c(8): warning C4019: empty statement at global scope
> 1>c:\products\source\scratch10\ctest.c(12): warning C4019: empty statement at global scope
> 
> The help for the error code:
> 
> 
> <https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-4-c4019?view=msvc-170> 
> 
> 
>>
>> With "/W4", or even "/Wall", it still doesn't diagnose a stray semicolon
>> at file scope.� (I wouldn't expect a warning option to be the
>> incantation that makes the compiler conform to the standard.)
> 
> Eh?� On my system I got the above messages for stray semi-colons.� I'm using VS2017.

Oh right - I also had /Za set, and it seems it needs both /W4 and /Za.  I don't have any /std: 
option as it doesn't seem to be valid for VS2017.

> 
>>
>> The "/Za" option is supposed to disable language extensions, but it
>> complains that "'/Za' and '/std:c17' command-line options are
>> incompatible".
>>
>> The implementation supports both C and C++.� It seems to treat C as a
>> second-class citizen.� (I think, but I'm not sure, that a stray
>> semicolon at file scope is legal in C++; it's called an
>> "empty-declaration".)
>>
>>> If it doesn't diagnose at its maximum warning level, then okay, ~I
>>> lose the syntax battle.
>>
>> I'd say that Microsoft's compiler loses the syntax battle.
>>
>> [Remainder read and snipped]
>>